1. Field of the Invention
The present invention relates to a stator of a rotating electric machine, such as an alternating current (AC) generator or an AC motor, in which coils are concentratedly wound on a plurality of teeth of a stator core.
2. Description of the Background Art
A concentrated-winding type stator of a rotating electric machine includes a stator core which is formed by stacking a plurality of core segments to a specified thickness and joining the core segments by pressure joining or welding and coils concentratedly wound in slots in the stator core. Each of the core segments has multiple teeth extending radially inward from a back yoke and slots formed between the adjacent teeth. These core segments are made by punching an electromagnetic steel sheet by a press, for instance.
Conventionally, a coil is wound around each tooth one layer on top of another by use of a winding machine which pays out a conductor through a nozzle while causing the nozzle to pass along slots between adjacent teeth. To ensure that the nozzle will not go into contact with any nearby coils during winding operation, it is necessary to make available a sufficient space between the nozzle and the nearby coils. This requirement however causes a decrease in space factor of winding in the slots, that is, the ratio of the cross-sectional area of the coil in each slot to the cross-sectional area of the slot, eventually leading to an increase in the size of the rotating electric machine.
A previously known approach to improving the space factor of winding is found in Japanese Patent Application Publication No. 1998-146030. In a first arrangement proposed in this Publication, two coils are wound in each slot, one coil in an inner layer and the other coil in an outer layer. In a second arrangement proposed in the Publication, each coil is wound in two adjacent slots in such a fashion that each coil is laid in an inner layer in a slot on one side of a tooth and in an outer layer in a slot on the other side of the tooth.
In the aforementioned first conventional arrangement in which two coils, or parallel straight portions of a pair of adjacent coils, are placed in the inner and outer layers of each slot, the coils in the inner and outer layers differ from each other in resistance, inductance and cooling efficiency. This could produce such problems as fluctuations in output torque and other output characteristics as well as acoustic noise.
In the aforementioned second conventional arrangement in which each coil is wound in the inner layer in the slot on one side of a tooth and in the outer layer in the adjacent slot on the other side of the tooth, the coils may be wound one after another such that a straight portion of one coil fitted in the outer layer lies on a straight portion of the adjacent coil fitted in the inner layer. This second arrangement however has a problem that it is extremely difficult to wind the last one of the coils. This is because when the last coil is wound, the outer layer of the last slot, in which a straight portion of the last coil is to be wound in the inner layer, is already occupied by a straight portion of the coil wound around the immediately adjacent tooth.